This invention relates to nematic liquid crystal compositions comprising a liquid crystalline compound as a main component. More particularly, the invention relates to liquid crystal compositions for an active-matrix liquid crystal display device and liquid crystal display devices using these liquid crystal compositions. For convenience, a liquid crystalline compound may be abbreviated hereafter to a compound, and a liquid crystal composition to a composition.
Liquid crystal display devices (LCD) easily permit low power consumption and a lightweight, planar, flat surface, as compared with CRT (cathode-ray tube mode display). Therefore, a variety of LCD have been put to practical use, including a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a thin-film transistor (TFT) mode and the like. Among them, active-matrix LCD (AM-LCD) such as TFT or the like has been advanced in the high definition and high view angle, attracting the attention as the favorite of flat display panels.
The following general properties (1) to (4) are required for the liquid crystal compositions for AM-LCD.
(1) The specific resistance and voltage holding ratio are high for achieving a better contrast of LCD.
(2) The temperature range of nematic phase is broad for enabling the outdoor use of LCD. That is, the upper limit of the temperature exhibiting nematic phase is high, while the lower limit is low.
(3) The threshold voltage is low for reducing a consumption power of LCD.
(4) The viscosity is low for increasing the response rate of LCD.
Japanese Patent No. 3,086,228, WO91/13947 (Japanese Patent Hyo Hei 4-505477 (JP-A-4-505477)), Japanese Patent Kokai Hei 5-105876 (JP-A-5-105876), etc. disclose the compositions usable in AM-LCD. These compositions, however, show the tendency that the nematic phase range is narrow, the viscosity is high and the threshold voltage is high, and therefore they are not suitable for use in AM-LCD. The components for the composition have been investigated in accordance with the intended use, but more improvement is always required.
An object of the invention is to provide a liquid crystal composition, especially having broad nematic phase range, low viscosity, low threshold voltage and large pre-tilt angle, while meeting the general properties required for AM-LCD.
Another object of the invention is to provide a liquid crystal display device composed of the liquid crystal composition.
We have studied various liquid crystal compositions comprising liquid crystalline compounds in an effort to solve the above-mentioned problems, and as a result the above objects could be accomplished by the present invention as mentioned below.
In a first aspect, the present invention relates to a liquid crystal composition which comprises Component I comprising at least one compound selected from the compounds of the formulas (I-1) to (I-3), Component II comprising at least one compound selected from the compounds of the formulas (II-1) to (II-4), Component III comprising at least one compound selected from the compounds of the formulas (III-1) and (III-2), and Component IV comprising at least one compound selected from the compounds of the formulas (IV-1) to (IV-3). 
wherein R and Rxe2x80x2 each independently represent an alkyl group of 2 to 7 carbon atoms, Y independently represents H or F, and n represents an integer of 0 to 4.
0.1 to 3% by weight of an optically active material may be added to the composition, based on the total weight thereof.
In an embodiment of the first aspect, Component I comprises the compound of the formula (I-1), the compound of the formula (I-2), or the compound of the formula (I-3).
In another embodiment of the first aspect, the content of Component I is 5 to 30% by weight, the content of Component II is 30 to 70% by weight, the content of Component III is 5 to 45% by weight, and the content of Component IV is 5 to 30% by weight, based on the total weight of the composition.
In a second aspect, the present invention relates to a liquid crystal composition which consists essentially of four components of Component I comprising at least one compound selected from the compounds of the formulas (I-1) to (I-3), Component II comprising at least one compound selected from the compounds of the formulas (II-1) to (II-4), Component III comprising at least one compound selected from the compounds of the formulas (III-1) and (III-2), and Component IV comprising at least one compound selected from the compounds of the formulas (IV-1) to (IV-3), 
in which R and Rxe2x80x2 each independently represent an alkyl group of 2 to 7 carbon atoms, Y independently represents H or F, and n represents an integer of 0 to 4.
This composition may contain impurities such as by-products produced during the synthesis of each compound. So far as the objects of the present invention are achieved, several percent of liquid crystalline compounds may be added to the composition. In addition to the liquid crystalline compounds, small amounts of additives may be added to the composition, if necessary.
0.1 to 3% by weight of an optically active material may be added to the composition, based on the total weight thereof.
In an embodiment of the third aspect, the content of Component I is 5 to 30% by weight, the content of Component II is 30 to 70% by weight, the content of Component III is 5 to 45% by weight, and the content of Component IV is 5 to 30% by weight, based on the total weight of the composition.
In a third aspect, the present invention relates to a liquid crystal composition of the first aspect, which further contains Component V comprising at least one compound selected from the compounds of the formulas (V-1) and (V-2), 
wherein R independently represents an alkyl group of 2 to 7 carbon atoms.
0.1 to 3% by weight of an optically active material may be added to the composition, based on the total weight thereof.
In an embodiment of the second aspect, the content of Component I is 5 to 30% by weight, the content of Component II is 30 to 70% by weight, the content of Component III is 5 to 45% by weight, the content of Component IV is 5 to 30% by weight, and the content of Component V is 5 to 25% by weight, based on the total weight of the composition.
In a fourth aspect, the present invention relates to a liquid crystal display device in which each of the liquid crystal compositions of the above-mentioned aspects is used.
In an embodiment of the fourth aspect, the liquid crystal composition used is that of the first aspect.
In another embodiment, the liquid crystal composition used is that of the second aspect.
In a further embodiment, the liquid crystal composition used is that of the third aspect.
Component I in the composition of the invention is the compounds of the formulas (I-1) to (I-3). The formula (I-2) is specified as follows: 
Component I has an optical anisotropy (xcex94n) of about 0.02 to 0.07, a clearing point (Tc) of about xe2x88x9230 to 20xc2x0 C. and a dielectric anisotropy (xcex94 xcex5) of 3 to 8, and shows excellent thermal and chemical stabilities and excellent solubility. Component I reduces the lower limit of nematic phase range of the composition, while lowering the viscosity. However, the composition comprising Component I only tends to have a reduced pre-tilt angle, in addition to an exceedingly lowered upper limit of nematic phase range.
Component II is the compounds of the formulas (II-1) to (II-4). The formula (II-1) is specified as follows: 
Component II has Tc of about 50 to 120xc2x0 C., xcex94 xcex5 of about 5 to 10 and xcex94n of about 0.07 to 0.14, and shows excellent thermal and chemical stabilities and excellent solubility. Component II increases the upper limit of nematic phase range of the composition, while reducing the threshold voltage. However, the composition comprising Component II only tends to have an increased lower limit of nematic phase range.
Component III is the compounds of the formulas (III-1) and (III-2). The formulas (III-1) and (III-2) are specified as follows: 
Component III has Tc of about 100 to 160xc2x0 C., xcex94 xcex5 of about 4 to 10 and xcex94n of about 0.07 to 0.10, and shows excellent thermal and chemical stabilities and excellent solubility. Component III reduces the threshold voltage of the composition. However, the composition comprising Component III only tends to have an increased viscosity, in addition to the increased lower limit of nematic phase range.
Component IV is the compounds of the formulas (IV-1) to (IV-3). Component IV has Tc of about 60 to 230xc2x0 C., xcex94 xcex5 of approximately 0 and xcex94n of about 0.03 to 0.11, and shows excellent thermal and chemical stabilities and excellent solubility. Component IV increases the pre-tilt angle of the composition to broaden the nematic phase range. However, the composition comprising only Component IV only tends to have an increased threshold voltage.
Combination of Components I, II, III and IV can prepare liquid crystal compositions for AM-LCD which have broad nematic phase range, low viscosity, low threshold voltage and large pre-tilt angle.
The content of Component I is preferably 5 to 30% by weight, more preferably 5 to 25% by weight, based on the total weight of the composition. Less than 5% by weight may increase the lower limit of nematic phase range of the composition. More than 30% by weight may reduce the upper limit of the nematic phase range.
The content of Component II is preferably 30 to 70% by weight, more preferably 35 to 65% by weight, based on the total weight of the composition. Less than 30% by weight may increase the threshold voltage. More than 70% by weight may increase the lower limit of nematic phase range.
The content of Component III is preferably 5 to 45% by weight, more preferably 5 to 40% by weight, based on the total weight of the composition. Less than 5% by weight may increase the threshold voltage. More than 45% by weight may increase the lower limit of nematic phase range.
The content of Component IV is preferably 5 to 30% by weight, more preferably 5 to 25% by weight, based on the total weight of the composition. Less than 5% by weight may increase the threshold voltage. More than 30% by weight may increase the lower limit of nematic phase range.
The content of Component V is preferably 5 to 25% by weight, based on the total weight of the composition.
The liquid crystal display device of the invention has preferably the use temperature range of from at least 70xc2x0 C. to xe2x88x9220xc2x0 C. If this range is narrower, the display of device may be difficult outdoors. For this reason, it is desirable that the upper limit temperature exhibiting the nematic phase of the composition is 70xc2x0 C. or higher and the lower limit temperature is xe2x88x9220xc2x0 C. or lower.
The compounds of the formulas (I-1) to (IV-3) can be synthesized by the methods disclosed in the following Japanese patent publications. The method for the synthesis of the compounds of the formula (I-1) is disclosed in Japanese Patent Kokai Sho 57-2226 (JP-A-57-2226). As an example of the compounds of the formula (I-2), the method for the synthesis of the compounds of the formula (I-2-1) is disclosed in Japanese Patent Kokai Sho 59-10533 (JP-A-59-10533). The method for the synthesis of the compounds of the formula (I-3) is disclosed in Japanese Patent Kokoku Sho 61-21937 (JP-B-61-21937). As an example of the compounds of the formula (II-1), the method for the synthesis of the compounds of the formula (II-1-1) is disclosed in Japanese Patent Kokai Sho 57-64626 (JP-A-57-64626). The method for the synthesis of the compounds of the formula (II-2) is disclosed in Japanese Patent No. 2674853. The method for the synthesis of the compounds of the formula (II-3) is disclosed in Japanese Patent Kokai Sho 57-154135 (JP-A-57-154135). The method for the synthesis of the compounds of the formula (II-4) is disclosed in Japanese Patent Kokai Sho 61-282328 (JP-A-61-282328).
As an example of the compounds of the formula (III-1), the method for the synthesis of the compounds of the formula (III-1-1) is disclosed in Japanese Patent Kokoku Sho 61-26979 (JP-B-61-26979). As an example of the compounds of the formula (III-2), the method for the synthesis of the compounds of the formula (III-2-1) is disclosed in Japanese Patent Kokoku Sho 60-59893 (JP-B-60-59893). The method for the synthesis of the compounds of the formula (IV-1) is disclosed in Japanese Patent Kokoku Hei 4-30382 (JP-B-4-30382). The method for the synthesis of the compounds of the formula (IV-2) is disclosed in Japanese Patent Kokoku Hei 4-30382 (JP-B-4-30382). The method for the synthesis of the compounds of the formula (IV-3) is disclosed in Japanese Patent Kokai Sho 57-21083 (JP-A-57-21083). The compounds of the formulas (V-1) and (V-2) can be readily synthesized by a person skilled in the art in accordance with known methods for the synthesis. As mentioned above, the compounds for Components I to V can be synthesized in view of the prior art references. The compounds for each component are mixed, heated and dissolved for each other to prepare a composition.
In any case, small amounts of additives may be added to the compositions of the present invention, if necessary. For the purpose of inducing a helical structure of liquid crystalline molecule to provide a twist angle, a chiral dopant (optically active compound) such as CM-33 and cholesteric nonanoate (CN) or the like may be added to the compositions. This is because a twist angle of about 90 degrees and 180 to 260 degrees is required for TN and STN modes, respectively. 
The compositions of the invention can also be used in OCB (Optically Controlled Bend) mode and IPS (In Plain Switching) mode. In these cases, no chiral dopant is added usually. Further, the compositions of the invention can be used for guest-host mode, by adding a dichronic dye such as merocyanines, styryls, azo, azomethines, azoxy, quinophthalones, anthraquinones or tetrazines.
The liquid crystal compositions of the invention can also be used in a polymer dispersed liquid crystal display device, and for a birefringence controlled mode and a dynamic scattering mode.